The present invention relates to acicular goethite particles and acicular magnetic iron oxide particles which have a uniform particle size distribution, are substantially free of dendrites, and have a large aspect ratio (major axial diameter/minor axial diameter) and an excellent coercive force distribution.
With progressing miniatirization and weight-reduction of magnetic recording and reproducing apparatuses in recent years, the necessity for a recording medium having a higher performance such as a magnetic tape and a magnetic disk has been increasing more and more. In other words, a magnetic recording medium is required to have a higher recording density, higher sensitivity and higher output characteristic. The magnetic characteristics of magnetic particles which are demanded in order to satisfy the above-mentioned requirements for the magnetic recording medium, are a high coercive force and an excellent dispersibility. In order to improve the sensitivity and the output of a magnetic recording medium, the magnetic particles must have as high a coercive force as possible. this fact is described in, for example, DEVELOPMENT OF MAGNETIC MATERIALS AND TECHNIQUE OF IMPROVING THE DISPERSION PROPERTY OF MAGNETIC POWDER (1982), published by K. K. Sogo Gijutsu Kaihatsu Center, p. 310: "Since the improvement of magnetic tapes has been directed toward a higher sensitivity and a higher output, it is an important point to enhance the coercive force of acicular .gamma.-Fe.sub.2 O.sub.3 particles, . . . "
In order to improve the recording density of a magnetic recording medium, the magnetic recording medium must have a high coercive force and a large residual magnetization (Br), as described in the said DEVELOPMENT OF MAGNETIC MATERIALS AND TECHNIQUE OF IMPROVING THE DISPERSION PROPERTY OF MAGNETIC POWDER. p. 312: "The condition for high-density recording in a coating-type tape is that it is possible to keep the high output characteristics with respect to a short-wavelength signal at a low noise level. For this purpose, it is necessary that both the coercive force (Hc) and the residual magnetization (Br) are large, and that the thickness of the coating film is thin". It is therefore necessary that the magnetic particles have a high coercive force and they are excellent in dispersibility in the vehicle, and orientation property and packing density in the coating film.
In order to enhance the output of a magnetic recording medium, it is required to have a small switching field distribution (hereinunder referred to as "S.F.D.") and hence, the magnetic particles are required to have a small width of coercive force distribution. This fact is described in Japanese Patent Application LaidOpen (KOKAI) No. 63-26821 (1988): "FIG. 1 is a graph showing the relationship between the S.F.D. of the above-described magnetic disk and the recording and reproducing output. . . . The relationship between the S.F.D. and the recording and reproducing output is linear, as is clear from FIG. 1. It indicates that the recording and reproducing output is enhanced by using ferromagnetic powder having a small S.F.D. That is, in order to obtain a large output, an S.F.D. of not more than 0.6 is necessary.
As well known, the coercive force of magnetic iron oxide particles depend upon the configurational anisotropy, crystalline anisotropy, strain anisotropy, exchange anisotropy, or the interaction thereof.
Acicular magnetite particles and acicular maghemite particles which are used as magnetic iron oxide particles at present show a relatively high coercive force by utilizing the anisotropy derived from their shapes, namely, by increasing the aspect ratio (major axial diameter/minor axial diameter).
The known acicular magnetic particles are obtained by reducing as a starting material goethite particles or hematite particle obtained by heat-treating the goethite particle at 280.degree. to 450.degree. C. in a reducing gas such as hydrogen, to form magnetite particles, or by further oxidizing the thus-obtained magnetite particles at 200.degree. to 400.degree. C. in air to form maghemite particles.
The known acicular magnetic iron oxide particles modified with Co or Co and Fe are obtained by dispersing acicular magnetite particles or acicular maghemite particles as the precursor particles in an alkaline suspension containing cobalt hydroxide or an alkaline suspension containing cobalt hydroxide and ferrous hydroxide so that 0.1 to 15.0 atomic% of Co based on Fe of the precursor particles is contained, and heat-treating the resultant dispersion.
The residual magnetization (Br) in a magnetic recording medium depends upon the dispersibility of the magnetic particle in the vehicle, and the orientation property and packing density of the magnetic particles in the coated film, and in order to improve these properties, the magnetic particles to be dispersed in the vehicle are required to have as large an aspect ratio (major axial diameter/minor axial diameter) as possible, a uniform particle size distribution and no inclusion of dendrites.
As described above, magnetic iron oxide particles which have a substantially uniform particle size distribution, which are substantially free of dendtritess and which have a large aspect ratio (major axial diameter/minor axial diameter) are now in the strongest demand. In order to obtain magnetic iron oxide particles provided with these properties, it is necessary that as the starting material goethite particles have a substantially uniform particle size, are substantially free of dendrites and have a large aspect ratio (major axial diameter/minor axial diameter). e.g. an aspect ratio of not less than 20.
As a method of producing goethite particles which are the starting material, the following methods are conventionally known: (1) a method of producing acicular goethite particles by oxidizing a suspension containing colloidal ferrous hydroxide which is obtained by adding not less than one equivalent of an aqueous alkali hydroxide solution to a ferrous salt solution, at a temperature of not more than 80.degree. C. and pH of not less than 11 by blowing an oxygen-containing gas into the suspension (Japanese Patent Publication No. 39-5610 (1964)); (2) a method of producing spindle-shaped goethite particles by oxidizing a suspension containing FeCO.sub.3 which is obtained by reacting an aqueous ferrous salt solution with an aqueous alkali carbonate solution, by blowing an oxygen-containing gas into the suspension (Japanese Patent Application Laid-Open (KOKAI) No. 50-80999 (1975)); and (3) a method of producing acicular goethite nucleus particles by oxidizing an aqueous ferrous salt solution containing a colloidal ferrous hydroxide or an iron carbonate which are respectively obtained by adding not more than one equivalent of an aqueous alkali hydroxide solution or an aqueous alkali carbonate solution to an aqueous ferrous salt solution, by blowing an oxygen-containing gas into the suspension, and then growing the goethite nucleus particles by adding to the thus-obtained aqueous ferrous salt solution containing the goethite nucleus particles not less than one equivalent of an aqueous alkali hydroxide solution based on Fe.sup.2 + in the aqueous ferrous salt solution and blowing an oxygen-containing gas into the resultant aqueous ferrous salt solution for oxidization (Japanese Patent Publication No. 59 -48766 (1984), Japanese Patent Application Laid-Open (KOKAI) Nos. 59-128293 (1984), 59-128294 (1984), 59-128295 (1984) and 60-21818 (1985)).
Although acicular magnetic iron oxide particles which have a uniform particle size distribution, which are substantially free of dendtritess, and which have a large aspect ratio. (major axial diameter/minor axial diameter) and excellent coercive force distribution are now in the strongest demand, the particles obtained by the method (1) of producing as a starting material goethite particles contain dendrites and cannot be said to have a uniform particle size distribution in-spite of the large aspect ratio (major axial diameter/minor axial diameter), particularly, an aspect ratio of not less than 10.
According to the method (2), although spindle-shaped particles having a uniform particle size distribution and being free of dendrites are produced, the aspect ratio (major axial diameter/minor axial diameter) thereof is not more than about 7. That is, the method (2) is defective in that it is difficult to produce particles having a large aspect ratio (major axial diameter/minor axial diameter). This phenomenon tends to be more prominent as the major axial diameter of the particles become smaller. Various attempts have been made to increase the aspect ratio (major axial diameter/minor axial diameter) of spindle-shaped goethite particles, but the actual aspect ratio (major axial diameter/minor axial diameter) obtained has been not more than about 17 to 18, which cannot be said satisfactory.
The object of the method (3) is to improve the properties such as particle size, aspect ratio (major axial diameter/minor axial diameter) and the presence or absence of dendrites of the acicular goethite particles produced by the method (1) or (2), but goethite particles produced by the method (3) can not be said to satisfy the demand for various properties have not been obtained yet.
Therefore, acicular magnetic iron oxide particles produced from as the starting material these goethite particles cannot be said to have a uniform particle size distribution, to be free of dendrites and to have a large aspect ratio (major axial diameter/minor axial diameter).
Accordingly, it is an object of the present invention to provide acicular goethite particles and acicular magnetic iron oxide particles which have a uniform particle size distribution, which are substantially free of dendrites, and which have a large aspect ratio (major axial diameter/minor axial diameter) and excellent coercive force distribution.
As a result of studies undertaken by the present inventors, it has been found that by blowing an oxygen-containing gas into a ferrous salt reaction solution containing colloidal ferrous hydroxide or iron-containing colloidal precipitates which is obtained by reacting an aqueous ferrous salt solution with less than one equivalent of an aqueous alkali hydroxide solution and/or an aqueous alkali carbonate solution based on Fe.sup.2 + in the aqueous ferrous salt solution so as to oxidize the colloidal ferrous hydroxide or iron-containing colloidal precipitates and to produce acicular goethite nucleus particles, adding to the resultant aqueous ferrous salt reaction solution containing the acicular goethite nucleus particles not less than one equivalent of an aqueous alkali carbonate solution based on Fe.sup.2 + in the aqueous ferrous salt reaction solution, and blowing an oxygen-containing gas into the mixed aqueous ferrous salt reaction solution so as to grow the goethite nucleus particles, the thus-obtained acicular goethite particles have a uniform particle size distribution, are substantially free of dendrites, and have a large aspect ratio (major axial diameter/minor axial diameter) and excellent coercive force distribution. On the basis of this finding, the present invention has been achieved.